3(M-2N)
=3[2a^2+3ab-2a-1-2(a^2+ab-1)]
=3[2a^2+3ab-2a-1-2a^2-2ab-2]
=3[ab-2a-3]
=3(b-2)a-9
结论与a无关,a前面的系数为0,
即b-2=0,
b=2.